The total internal reflection (TIR) if illumination light within a prism is a known method of directing illumination light on to a DMD (digital micro-mirror device) for use in reflecting that light as a projectable display. By selectively separating the light reflected from DMD pixels, one may control what light is projected, and when. A prism may be used in this way to exploit total internal reflection (TIR) to direct “selected” light and “deselected” light through different exit faces of the prism. This allows the projector system to keep “de-selected” light out of a subsequent projector lens.
The drawback with this is the required increase in build cost and complexity, especially since these prisms are bespoke optical elements. Another drawback occurs when multi-element prism assemblies are employed in an attempt to suitably direct light. In particular, this is the introduction of multiple air-gaps in the path downstream of the DMD, which degrade the projected image resolution. In general these should be minimised.
The present invention aims to address these matters with a compact, low-cost and lower-complexity optical design.